The invention relates to an apparatus for combining light emanating from a linear scanning field on a relatively small receiver via an optical arrangement producing a scanning light spot with a light ray bending device subject to the action of a light beam, a concave mirror extending in the scanning direction and a cylindrical lens parallel to the scanning field and extending in the vicinity of the latter, whereby the transmitting and receiving beams of rays are separated by pupil separation.
In general such apparatus function with a laser as the light source because it emits a narrow parallel beam which is particularly advantageous for producing a fine scanning light spot. In general the laser beam is passed via two crossed cylindrical lenses and an objective to a mirror wheel forming the light ray bending device. The function of the cylindrical lenses is to fan out the laser beam in such a way that it fully illuminates the individual surfaces of the mirror wheel. Advantageously the mirror wheel operates with 12 to 20, and preferably 16 mirror surfaces, which are uniformly distributed over the periphery. In principle it is also possible to use an oscillatory mirror as the light ray bending device.
From the mirror wheel the light beam is deflected via a lamellar plane mirror onto an also lamellar concave mirror, whose focal point or line is located approximately on the reflecting surface of the mirror wheel. The concave mirror directs the transmitting beam of rays which strikes it onto a parallel cylindrical lens located substantially at the spacing of its focal length from a surface on which a linear scanning light spot is to be produced. In general the surface is a web which is to be monitored for faults, such as a metal or paper web surface. The web appropriately moves perpendicular to the rectilinear scanning direction, so that a continuous line scanning of the web is ensured provided that the relationship between web speed and scanning speed is correctly chosen.
Although the receiving beam of rays emanating from the light spot produced on the web can be received and evaluated by a separate receiving device it is preferable for the receiving beam of rays to at least partly be returned in the same way as the transmitting beam of rays to a preferably photoelectrically operating receiving arrangement. The electrical signal emitted by the photoelectric receiver is then a measure of the state of the scanned surface. For example a change in the electrical signal reveals scratches or other faults on a metal surface or faults such as dark spots or holes in the case of a paper web.
If the optical elements of the transmitting beam power are to be used simultaneously for further conducting the receiving beam of rays reference is made to an autocollimating beam path. It is then advantageous for a good separation of the receiving and transmitting beams of rays to use the so-called pupil separation, which means that a part, e.g. a third of at least certain optical elements is used for further conducting the transmitting beam of rays, whilst the remainder, e.g. two-thirds is used for further conducting the receiving beam of rays.
An important problem with autocollimating beam paths is that on the one hand an adequately large light spot with the necessary dimensions is produced on the path for the receiving beams to be reliably reflected back onto the light ray bending device and on the other that there is a reliable spatial separation between the transmitting and receiving beams of rays in the area of the photoelectric receiver. Just as much importance must be attached in this connection to a simple and more particularly compact construction of the apparatus as to the extensive avoidance of susceptibility to vibrations or displacements after relatively long periods. In particular it must be ensured that too much receiving light is not lost in the receiving beam path, so that at the photoelectric receiver there is still a quantity of light which clearly exceeds the background noise for evaluation purposes.